The present invention relates to a relatively high molecular weight polyvinyl sulfonate and to a process for inhibiting scale deposition, particularly inorganic sulfate such as barium sulfate, in a subterranean formation utilizing an aqueous solution having the relatively high molecular weight polyvinyl sulfonate dissolved therein.
Precipitation of inorganic salts, such as calcium carbonate and calcium, barium, and strontium sulfate, as scale deposits is a persistent and common problem encountered in many field operations for the recovery of hydrocarbons from subterranean formations. Commingling of incompatible aqueous fluids during field operations, especially enhanced oil recovery (EOR) operations involving a waterflood or water drive, results in scale formation and deposition in the formation and in production equipment and tubing. Two or more aqueous fluids are incompatible if each fluid contains distinct ions which form a precipitate and deposit as a scale when the two or more aqueous fluids are commingled. Usually the connate water or brine present in a reservoir will contain barium, calcium and/or strontium ions while water injected into the subterranean formation during EOR operations will contain sulfate ions. For example, offshore operations may involve the injection of large volumes of sea water containing a relatively large concentration of sulfate ions into a subterranean formation having brine containing relatively large concentrations of barium, calcium, and strontium. Upon mixing of the aqueous fluids in situ, precipitation of barium, calcium, or strontium sulfate will occur in the formation and the subsurface and surface production equipment and/or tubing. Commingling of incompatible aqueous fluids usually occurs within the near production well bore environment of a subterranean formation.
Injection of carbon dioxide into a subterranean hydrocarbon-bearing formation as an EOR method results in absorption of carbon dioxide by connate water present in the formation. In addition, some subterranean formation brines, such as those found in the North Sea, may naturally contain a relatively large concentration of carbon dioxide. As pressure is reduced, for example during production, carbon dioxide flashes off to the gas phase thereby increasing the pH of the aqueous fluids and permitting formation of calcium carbonate scale predominantly in the near production well bore environment of the formation and in subsurface and surface production equipment and/or tubing.
Conventional removal of scale formed within a subterranean formation and the subsurface and surface production equipment and tubing is costly and ineffective. Scale removal by repeated injection of a chemical agent is relatively expensive. Thus, scale has been removed utilizing various mechanical devices, such as impact jets and/or cavitation jets. As the downtime associated with pulling production tubing and cleaning such tubing above ground is expensive, particularly in offshore locations, wells are cleaned downhole after killing the well. Such mechanical cleaning is timeconsuming, relatively inefficient, and potentially hazardous where a radioactive precipitate, for example radium sulfate, is present in the scale to be removed.
Inhibition of scale deposition has been advanced as a more feasible, and accordingly preferable, approach to effectively reducing scale deposition. Conventional commercial scale inhibitors consist primarily of polyelectrolytes, such as polycarboxylates or polyphosphonates. However, the effectiveness of such polyelectrolyte scale inhibitors significantly depends upon the degree of ionization of these inhibitors at the pH value of connate waters. At relatively low pH values, e.g. equal to or less than about 6.0, the effectiveness of a conventional polyelectrolyte scale inhibitor to inhibit barium, calcium, or strontium sulfate scale formation significantly decreases. In addition, conventional polyelectrolyte scale inhibitors utilized to inhibit inorganic sulfate scale dissolve calcium carbonate scale thereby increasing calcium ion concentration which causes undesirable precipitation of the conventional polyelectrolyte scale inhibitors. Thus, a need exists for a scale inhibitor for use in subterranean formations which effectively inhibits the formation of scale, particularly inorganic sulfate such as barium sulfate, in relatively low pH environments, such as a pH of about 6.0 and below.
Accordingly, it is an object of the present invention to provide a novel scale inhibitor which will effectively inhibit the formation of scale, particularly inorganic sulfate such as barium sulfate, in aqueous fluid present in and/or produced from a subterranean formation.
It is another object of the present invention to provide a scale inhibitor which does not dissolve carbonate scale to any appreciable degree when injected into a subterranean formation via a well bore in fluid communication therewith.
It is a further object of the present invention to provide a process for inhibiting scale formation, particularly inorganic sulfate such as barium sulfate, from fluids having a pH equal to or less than 6.0 which are present in and/or produced from a subterranean formation.